Kudosu is awarded to mod posts that are helpful and point out useful suggestions for the beatmap. If a mapper believes that a mod post is useful and addresses a valid issue, they can click the thumbs up button and award kudosu to the user who posted the mod.
Nowadays, kudosu is used for the evaluation of a user's capability to become a Beatmap Nominator. A user must have 200 kudosu in osu! or 150 kudosu in osu!taiko, osu!catch, or osu!mania in order to apply.
The word kudosu, sometimes stylised as kudosu!, is a portmanteau of kudos and osu!.